In a continuous multi-jet ink jet printer, a row of continuous ink jets are stimulated to produce a two-dimensional array of ink drops that are directed to a print receiving medium. The individual ink drops are selectively charged by a row of drop charging electrodes located adjacent the ink jets. Depending on the charge applied to the drops, they are deflected along one or more print trajectories to arrive at the print receiving medium, or they are deflected along a catch trajectory to be caught by a drop catcher. In high resolution ink jet printers, two types of drop charging inaccuracies can occur. One type, called drop-to-drop crosstalk, results from the influence of previously charged drops on subsequent drops in a single ink jet. The other type, called jet-to-jet crosstalk, results from the influence of the charge on neighboring electrodes. These two types of charging inaccuracies cause variations in drop placement on the print receiving medium, and hence a deterioration in the characters formed by the ink jet printer.
One approach to solving this crosstalk problem is described in published European Patent Application No. 0 104 951, published Apr. 4, 1984 by the Mead Corporation. According to this approach, a pattern of guard drops, charged to a catch potential, are provided to isolate the print drops from the influence of other print drops. According to the method, one or more guard drops are provided between consecutive print drops in each column to reduce drop-to-drop crosstalk, and one or more guard drops are provided between print drops in each row to reduce jet-to-jet crosstalk. The print drops are either printed or caught depending upon tne pattern to be produced; the guard drops are always caught. FIG. 2a shows an example of this technique applied to an ink jet print head having a single row of ink jets. In FIG. 2a, a portion of an ink jet print head 10 is shown schematically, with a plurality of ink jets 12 issuing therefrom. The ink jets break into drops 14, and the drops 14 are selectively charged by electrodes, not shown, to provide print drops, indicated by O's, and a pattern of guard drops, indicated by X's. Two guard drops are provided between consecutive print drops in both the row and column directions.
To print with such an ink jet print head, the print receiving medium is moved relative to the row of ink jets in a direction perpendicular to the row and at such a velocity that the successive print drops from each jet slightly overlap. An example of the resulting print is shown schematically in FIG. 2b. The printed drops 16 in each column are spaced apart by a distance d to provide the desired overlap between the print drops. As seen in FIG. 2b, the top and bottom edges of the print sample have a more ragged edge than the sides, due to tne provision of the guard drops in the drop pattern. In this example, the raggedness in the top and bottom edges has an amplitude of 2/3 d.
Thus, although the use of guard drops has reduced the inaccuracies in drop placement caused by jet-to-jet and drop-to-drop crosstalk, the guard jet method introduces a new source of raggedness in the edges of characters produced by the ink jet printer.
It is therefore the object of the present invention to provide a method and apparatus for producing guard drops in an ink jet printer that reduces the visibility of this edge raggedness.